Ultra Wideband (UWB) technology uses base-band pulses of very short duration to spread the energy of transmitted signals very thinly from near zero to several GHz. This technology is presently in use in military applications. Commercial applications will soon become possible due to a recent Federal Communications Commission (FCC) decision that permits the marketing and operation of consumer products incorporating UWB technology.
Presently, UWB is under consideration by the Institute of Electrical and Electronic Engineers (IEEE) as an alternative physical layer technology. See IEEE Standard 802.15.3a, which is designed for home wireless audio/video systems. This standard sets forth that UWB systems should operate well in an environment of uncoordinated piconets. Piconets, sometimes referred to as personal area networks (PANs), are formed when at least two devices, such as a portable PC and a cellular phone, connect.
Packet error rates (PER) can be attributed to narrow band interference (NBI) and to collision of-packets (i.e., symbols or information bits) transmitted on common communication (e.g., frequency) bands. “Multi-band” modulation technologies have been developed for UWB communication systems to deal with NBI. In multi-band UWB communication systems, the UWB frequency band is divided into multiple sub-bands utilizing a different spreading waveform in each sub-bands. One of the major advantages of the multi-band UWB system is its flexibility of working in environments with NBI. When NBI is detected, multi-band UWB systems may automatically shut down the corresponding sub-bands shared with the NBI to reduce the effect of NBI. Time/frequency hopping may be utilized in multi-band UWB systems to further reduce NBI effects.
FIG. 1 is a conceptual representation of a multi-band spectrum allocation for a UWB communication system which is in accordance with FCC mandates for such systems. The UWB spectrum of 7.5 GHz in the 3.1 GHz to 10.6 GHz frequency band is divided into 14 bands and each of bands 1-14 occupies 528 MHz of bandwidth. Bands 1-14 are grouped into band groups 1-5. For devices using UWB communications support for band group 1 is mandatory while it is optional for band groups 2-5.
FIG. 2A is a schematic diagram of a conventional superframe used for communications among a plurality of UWB devices in the UWB communication system.
FIG. 2B is an exemplary grouping of UWB devices. Although 3 UWB devices are shown any number of devices may be included in the UWB communication system.
Referring now to FIGS. 2A and 2B, because there is no central controller for piconet management, UWB devices A, B, and C from different but overlapping piconets coordinate themselves. Beaconing technology may be used for piconet management. Each UWB device A, B and C may transmit a respective beacon during a respective beacon slot S1-S3 and may listen to other UWB devices A, B and C for their beacons. Beacons from UWB devices A, B and C in a common area 20 may form a beacon group. When, for example, UWB device B joins an existing beacon group of UWB device A and C, its beacon is placed at the end of the beacon group in beacon slot S3. When, for example, UWB device A leaves the beacon group, other UWB devices B and C move their beacons forward to beacon slots S1 and S2, respectively, to make the beacon group as short as possible. Short beacon groups allow for more time in the super frame to allocate for data exchange.
The basic timing structure for data exchange is a superframe 200, 201 and 202. Superframe 200, 201 and 202 comprises (1) a beacon period (BP) 210, which is used to set timing allocations and to communicate management information for the piconet; (2) a priority channel access (PCA) period 220, which is a contention-based channel access that is used to communicate commands and/or asynchronous data; and (3) a distributed reservation protocol (DRP) period 230, which enables UWB devices A, B and C to reserve reservation blocks 240-1, 240-2 . . . 240-N outside of BP 210 of superframes 200, 201 and 202. DRP period 230 may be used for commands, isochronous streams and asynchronous data connections. Reservations made by UWB device A, B and C specify one or more reservation blocks 240-1, 240-2 . . . 240-N that UWB device A, B and C may use to communicate with one or more other UWB devices A, B and C on the piconet. UWB devices A, B and C using DRP period 230 for transmission or reception may announce reservations by including DRP Information Elements (IEs) in their beacons.
Each UWB device A, B and C may reserve an integral number of reservation blocks 240-1, 240-2 . . . 240-N (e.g., reservations are made in units of reservation blocks). UWB devices A, B and C may reserve multiple reservation blocks which may not be consecutive. That is, these multiple reservation blocks may have portions which are consecutive and other portions which are not consecutive. UWB devices A, B and C may reserve excess reservation blocks for error correction relevant retransmission and other control data, among others. Each UWB device A, B and C starts transmission at the beginning of a respective reserved reservation block.
Each reservation block 240-1, 240 . . . 240-N may include a plurality of frames 260 and may include intra-frame periods 270 and 280 such as MIFS periods, SIFS periods and a Guard period, among others. Conventionally, these intra-frame periods 270 and 280 are fixed duration periods, for example, typically, the MIFS period is 1.875 μs, the SIFS period is 10 μs, and the Guard period is 12 μs. These periods are not integer multiples of a symbol period.
UWB devices A, B and C may simultaneously transmit symbols (i.e., information bits) during frames 260 using Orthogonal Frequency Division Multiplexing (OFDM) modulation. Symbols may be interleaved across various bands to exploit frequency diversity and provide robustness against multi-path interference.
A simultaneously operating piconet (SOP) refers to, for example, multiple UWB devices A, B and C which may operate as different piconets in a common coverage area 20. When these devices A, B and C are used in apartment buildings, for example, the probabilities is high that multiple SOPs are operating. One major challenge for communication systems is dealing with interference caused by multiple SOPs that operate nearby.
FIG. 3 is a chart illustrating a conventional time-frequency code for band groups 1-4 illustrated in FIG. 1. For each band group 1-4, channels 1-7 may be established such that UWB device A may communicate over channel 1, UWB device B may communicate over channel 2 and UWB device C may communicate over channel 3. That is, for example, (1) in a first symbol period T1, UWB devices A, B, and C may communicate over frequency band 1; (2) in a second symbol period T2, UWB device A may communicate over frequency band 2, UWB device B may communicate over frequency band 3, and UWB device C may communicate over frequency band 1; (3) in a third symbol period T3, UWB device A may communicate over frequency band 3, and UWB devices B and C and may communicate over frequency band 2. Each channel may have a unique time/frequency hopping scheme, also referred to as a time-frequency code (TFC).
To support multiple SOPs and avoid interference, the information bits (i.e., symbols) are spread using the TFC. Typically, there are two types of TFCs used: ones in which symbols are interleaved over multiple bands, referred to as Time-Frequency Interleaving (TFI); and ones in which symbols are transmitted on a single band, referred to as Fixed Frequency Interleaving (FFI). Typically, each of the band groups 1-4 support both TFI and FFI.
For example, UWB devices communicating over channels 1-4 may use TFI, while UWB devices communicating over channels 5-7 may completely avoid collision by using FFI. However, because all symbols from one UWB device using, for example, channel 5 are transmitted on frequency band 1, total transmission power for frequency band 1 from the one UWB device is 4.7 dB higher than if distributed over frequency bands 1-3. Correspondingly, the FCC mandates that transmitters on channels 5-7, be required to reduce transmission power by 4.7 dB which results in a reduced coverage range.
A problem exists with the conventional communication systems that a plurality of UWB devices A, B and C may become unsynchronized due to the duration of reservation blocks 240-1, 240-2 . . . 240-N and these intra-frame periods 270 and 280. This can occur when UWB devices A, B and C start transmission at different reservation blocks. This can also occur even if previous frames 260 are synchronized with proper time-frequency codes and offsets and may cause increased collisions between communications from UWB devices A, B and C transmitted on common frequency bands.
What is needed is a communication system capable of synchronization of multiple UWB devices to increase system capacity.